The present invention relates generally to fastener driving tools employing magazines feeding fasteners to a nosepiece for receiving a driving force; and more specifically to such tools employing a fastener feeder mechanism powered with gas pressure generated during the fastener driving process.
Fastener driving tools, referred to here as tools or nailers, are known in the art and are powered by combustion, compressed gas (pneumatic), powder, and electricity. Portable fastener driving tools that drive collated fasteners disposed in a coil magazine are commercially available on the market and are manufactured by ITW Buildex, Itasca, Ill. The core operating principle of the tool and the respective fastener feeding mechanism is defined in ITW U.S. Pat. Nos. 5,558,264 and 7,040,521, both of which are incorporated by reference. In U.S. Pat. No. 5,558,264, a gas conduit is placed in fluid communication with the main drive cylinder of the power source.
Upon ignition and combustion, as the drive piston attached to the driver blade travels down the cylinder toward the fastener or nail to be driven, a supply of combustion gas is distributed into the gas conduit and is used to operate a spring-biased feeder mechanism. The gas pressure overcomes a biasing force provided by a spring, and causes movement of a feed piston located within a feed cylinder and connected to a feeding claw. Operationally associated with a strip of collated fasteners, the burst of compressed gas causes the feed piston and a linked feeding claw to retract and engage the next fastener in the strip. Next, upon dissipation of the combustion gas, the compressed spring expands, advances the feed piston and the next fastener toward the tool nosepiece for subsequent engagement with the driver blade.
In the '264 patent, the gas conduit is located in a wall of the drive cylinder and positioned between the drive piston's uppermost location (pre-firing position) and exhaust port openings located closer to an opposite end of the drive cylinder. The position of the conduit is such that a designated timing relationship is established during the drive cycle between the relative displacement of the drive piston and that of the feeder mechanism's feed piston. Such timing is an important design parameter for obtaining effective nail control and preventing nail jams within the nosepiece or the magazine. Optimally, the drive piston shears the nail from the collation media before the feed piston begins retraction, otherwise the nail will be driven with less control and an unsatisfactory nail drive can result. However, the mechanism of the '264 patent proved to be less reliable in that insufficient pneumatic power was supplied to the feed piston. The '521 patent disclosed moving the feed piston supply conduit inlet port directly in the combustion chamber to obtain a greater pneumatic force. A drawback of this arrangement is that the feed piston is actuated prematurely, causing misaligned fasteners in the tool nose as well as improperly driven fasteners.
Once the nail driving process is complete, a subsequent timing relationship between the return of the drive piston and advancement of the feeder mechanism is also important to obtain reliable piston return and nail feeding. The preferred timing scenario is for the drive piston to return to the pre-firing position before the feeder mechanism advances the nail into the tool nosepiece or nose (the terms are considered interchangeable). Currently, the feeder mechanism attempts to advance the nail into the nose while the drive piston and driver blade is returning to the pre-firing position. More specifically, the feed piston urges the next fastener toward the nosepiece prior to full retraction of the drive piston. This results in the nail being biased against the driver blade during the return cycle. See FIG. 6 and its associated description for timing diagram details. Between t2 and t3, the feed piston is urging the next fastener against the driver blade as the drive piston returns to its prefiring position. Only when the driver blade is fully retracted to its pre-firing position and a clear fastener passageway is provided does the fastener reach its drive position, indicated at t3. It should be understood that, referring to FIG. 6, as well as the other timing diagram in the application, that while tool state transitions are shown occurring instantaneously, there may be relative discrepancies or delays between steps.
The feeder mechanism includes a biasing spring that indirectly acts on the next nail to be driven, thereby exerting a transverse load component on the blade. The resulting friction prolongs the return of the driver blade, or even worse, prevents the driver blade from returning to the pre-firing position. When this occurs, the next fastener drive cycle does not result in a fastener being driven. This problem can be exacerbated by the amount of dirt, debris or collation media in the nose area of the tool.
Thus, there is a need for an improved fastener driver tool employing a method of establishing a preferred timing relationship between the drive piston and the advancement of the feeder mechanism during the return cycle of the drive piston.